Guanine Nucleotide Exchange Factors (GEFs) Have a Critical but Not Exclusive Role in Organelle Localization of Rab GTPases

Background: Rab localization has been ascribed to guanine nucleotide exchange factor (GEF) localization. Results: GEF deletions result in mislocalization of Rabs to other membranes, which can be bypassed by a Rab mutant. Conclusion: GEFs are critical for Rab localization, but Rabs also have a GEF-independent ability to localize correctly. Significance: Our data reveal that both GEFs and Rabs contribute to Rab localization in cells. Membrane fusion at eukaryotic organelles is initiated by Rab GTPases and tethering factors. Rabs in their GDP-bound form are kept soluble in the cytoplasm by the GDP dissociation inhibitor (GDI) chaperone. Guanine nucleotide exchange factors (GEFs) are found at organelles and are critical for Rab function. Here, we surveyed the overall role of GEFs in Rab localization. We show that GEFs, but none of the proposed GDI displacement factors, are essential for the correct membrane localization of yeast Rabs. In the absence of the GEF, Rabs lost their primary localization to the target organelle. Several Rabs, such as vacuolar Ypt7, were found at the endoplasmic reticulum and thus were still membrane-bound. Surprisingly, a Ypt7 mutant that undergoes facilitated nucleotide exchange localized to vacuoles independently of its GEF Mon1-Ccz1 and rescued vacuole morphology. In contrast, wild-type Ypt7 required its GEF for localization and to counteract the extraction by GDI. Our data agree with the emerging model that GEFs are critical for Rab localization but raise the possibility that additional factors can contribute to this process.